1. Field of the Invention
The present invention relates to a method and a bus device for transmitting safety-oriented data of a safety protocol in a field bus using an industrial Ethernet protocol.
2. Discussion of Background Information
Industrial Ethernet protocols, such as for example POWERLINK, Ethernet IP, ProfiNet, Ethercat, etc., are established in the automation field and are used for the standardized exchange of data between bus nodes in a field bus system. However, these protocols alone are not suitable for transmitting safety-related data, e.g. as defined by the international norm IEC 61508 or other safety-related norms. A safety protocol, such as for example OpenSAFETY, ProfiSafe, CIPsafety, Safety over Ethercat, etc., which safeguards the data in accordance with the safety requirements, is normally used for this purpose. The transmitted data are safeguarded against data errors, data loss and transmission errors by the safety protocol in order to be able to ensure correct transmission of the data within the intended transmission times. For this purpose, the safety protocol is provided with appropriate mechanisms, which enable possible transmission errors to be detected and corrected. In doing so, the data of the safety protocol are transmitted encapsulated in a conventional Ethernet message. Such industrial Ethernet protocols and safety protocols are sufficiently well-known, for which reason they will not be dealt with in more detail here.
Data communication protocols follow the well-known OSI layer model. Layers 1 and 2, that is to say the physical transmission layer and the data link layer, are the same for all industrial Ethernet protocols. As a basic principle therefore, all these industrial Ethernet protocols can be transmitted on the same Ethernet-based field bus. Layers 3 and 4, that is to say the network layer and the transport layer, are present in many standardized forms, such as TCP/IP or UDP/IP for example. As a rule, industrial Ethernet protocols use these standardized layers 3 and 4, but could also use dedicated specially developed layers 3 and 4. However, layers 5 and 6, that is to say the session layer and the presentation layer, are usually already specified and therefore fixed by the chosen industrial Ethernet. Every industrial Ethernet protocol is subject to a standardization committee which is responsible for specifying these layers 5 and 6. These layers 5 and 6 are therefore not accessible to users of the industrial Ethernet protocol. The software which is used to access the Ethernet bus, e.g. the automation software, runs in layer 7, the application layer. A safety protocol also runs in the application layer 7, i.e. all safety-oriented mechanisms of the safety protocol are implemented in the application layer 7 and safety-oriented data of the safety protocol are transmitted encapsulated in a message of the industrial Ethernet protocol. This is explained in FIG. 1 with reference to the OpenSAFETY safety protocol, which is transmitted with the Ethernet/IP industrial Ethernet protocol. OpenSAFETY is implemented in the application layer 7 where, for example, it uses the standardized CIP (Common Industrial Protocol) of the known Ethernet/IP protocol in layers 5 and 6. The safety-oriented message of the OpenSAFETY protocol is transmitted encapsulated in a TCP/IP or UDP/IP message via an Ethernet bus.
Because of the situation described above, it is possible for different safety-oriented protocols to be transmitted on the same Ethernet bus, as the layers 1 and 2 are identical. However, when transmitting safety-related data in different industrial Ethernet networks, it is necessary to make modifications in layers 5 and 6 and possibly also in layer 7 and/or to implement an intermediate layer for data conversion. Direct communication between bus nodes, which although they use the same safety protocol (e.g. OpenSAFETY) use different industrial Ethernet protocols (e.g. Ethernet/IP or PROFINET), is therefore likewise impossible. That is to say, a bus node, on which OpenSAFETY for Ethernet/IP is implemented, is unable to communicate with a node which has OpenSAFETY for PROFINET implemented. Flexibility in the application of safety protocols is therefore very restricted, or special gateways which connect different buses to one another must be provided, which in turn is laborious. Furthermore, a bus device which has a certain safety protocol implemented must be developed for every conceivable industrial Ethernet protocol, which leads to a multiplicity of variants of a bus device and requires considerable development and maintenance effort.
Added to this is the fact that safety protocols, such as OpenSAFETY for example, may have their own layers 5 and 6 implemented in order to implement the safety mechanisms. A safety-oriented message according to the chosen safety protocol is consequently packed in a secure data frame, which also includes information of layers 5 and 6 of the safety protocol. This secure message is then transmitted encapsulated in the chosen industrial Ethernet protocol, in which the secure message again contains information of layers 5 and 6 of the industrial Ethernet protocol. However, this results in an increased data overhead which understandably reduces the available data rate.